Claude Code (commander.js) refuses an unknown option before doing anything: prints "error: unknown option '<flag>'" and exits 1. Evidence from its own bug reports: anthropics/claude-code#62448 (claude respawn --all -> error: unknown option '--all', exit 1), #59371 (claude agents --mcp-config rejected), #79638 (mcp add passthrough). The visible cost of that policy is version skew: a wrapper (VS Code extension) passing a newer flag to an older CLI fails loudly (claude plugins --json -> unknown option). That is a loud failure, not a silent wrong action.
yoyo at 0977789a (Day 245): src/cli.rs warn_unknown_flags only eprintln!s 'Unknown flag X — ignored' and proceeds; e.g. yoyo --modle claude-haiku-4-5 -p hi reaches the API on the default (most expensive) model. Day 244 fixed invalid numeric VALUES (exit 2) but not unknown flags. Before refusing, census every flag yoyo's own scripts pass (evolve.sh is protected).